End group analysis of protein A indentified asparagine as the carboxy-terminal amino acid. No free amino terminal has been found, indicating that is is blocked. The selenium moiety of protein A has been identified as selenocysteine. It is the first essential selenium compound in a protein to be identified. In preliminary studies it was shown that cyanogen bromide cleavage produces a peptide containing selenocysteine. This will be isolated in larger amounts and sequenced so that the neighboring groups may be determined. Experiments will be conducted to ascertain if the selenium is introduced into Protein A as selenocysteine during protein sythesis or if it is added after the protein has been synthesized. The physiological role of selenium in the reaction that reduces glycine to acetate will also be investigated.